The invention relates to the film wind-up transmission of motion-picture cameras, particularly the type including a torque-equalizing clutch connected intermediate the drive motor and the drive wheel for the film wind-up reel.
Film wind-up transmissions of the type in question generally incorporate clutches which automatically disengage when the torque which they are transmitting reaches a value corresponding to the torque required to drive the film wind-up reel.
It is known to use friction clutches for this purpose. However, the value of the limit torque, the torque which when being transmitted by the friction clutch causes the clutch to automatically disengage, varies undesirably as a function of many external factors, such as humidity, temperature, aging, and the like. This variability of the value of the limit torque usually interferes greatly with proper operation of the camera. For example, image fluttering and wind-up malfunctions are two of the problems which may develop. Such friction clutches are particularly ill suited for the transmission of a uniform, maximum transmissible torque when the motion-picture camera in question is of the type designed to operate at a plurality of selectable, markedly different speeds. For example, switching over from 19 frames/second to 54 frames/second can produce changes in the value of the limit torque causing problems in the operation of the camera at both film speeds.
It is also known to make use of pawl motion-imparting mechanisms. When these are used for the standard film speed of 18 frames/second, or even possibly also 24 frames/second, the interfering factors mentioned above as detrimentally affecting the reliability of friction clutches are avoided. However, for higher film speeds such pawl mechanisms are not acceptable for practical reasons. Also, they are loud and excessively jerky in operation.
In general, both conventional friction clutches and pawl-type motion-imparting mechanisms are not satisfactory when, as a further requirement of operation, the motion-picture camera in which they are to be used is to be capable of operating with more than one type of film (e.g., silent film or sound film cassettes) requiring different respective film wind-up torques and accordingly different limit torques for the automatically disengaging clutches.
A friction clutch for film wind-up transmission is known whose limit torque can change in dependence upon the loading upon the clutch. In this construction, the pressure with which the frictional engaging parts of the clutch must be pressed together to establish a torque-transmitting engagement is limited by means of a fixed stop. Accordingly, with this construction, too, a changeover from one limit torque to another is not possible. Additionally, this particular construction exhibits all those disadvantages of friction clutches already discussed.